Recently, following the tendency of electronic devices towards high performance and compact size, a circuit board is required to have a high degree of lamination and high density. As a method of connecting layers within a substrate which can bond integrated circuits (IC) and components at the shortest distance, it is well-known that higher density can be attained through an inner-via-hole connection. In the case of a through-hole connection employed for a typical glass epoxy multilayer substrate, the connection is achieved by plating through-holes so that it was difficult to connect only the necessary layers. In addition, since the substrate comprises a top layer having an electrode land, this part can not be used to construct an electrode land of a surface mounting part. Therefore, due to these restrictions, the mounting density can not be easily enhanced.
In order to solve these problems, methods other than using through-holes are employed, for example, a method of reducing through-holes by disposing holes halfway in a substrate, or a method of improving mounting density by filling conductive paste in through-holes and by further closing up holes disposed in the top layer of a substrate at a plating step. However, due to the complicated manufacturing processes, the above-mentioned methods have cost and quantity production problems. On the other hand, simply the necessary layers can be connected in the inner-via-hole connection, and since there is no through-holes in the top layer of the substrate, the mounting property is also excellent.
When this connecting method is applied to a resin substrate, e.g. to a glass epoxy substrate, a double sided substrate establishes continuity by filling up through-holes with low-viscous solvent-type silver paste by means of a printing method and then dried and hardened. However, the specific resistance value of this connection is as high as about 10.sup.-3 .OMEGA..multidot.cm, and the reliability in heat and shock resistance such as in the heat cycle is poor.
In order to reduce the viscosity of conductive paste, conventional methods employed were to use large particles for reducing the amount of conductive filler or specific surface area, or to add a solvent with a low boiling point or a reactant type diluent.
However, when an additive amount of conductive filler was reduced, or when large particles were used, the problem occurred that contact points between fillers decreased and the resistance value of a via-hole-connection became so high that reliability could not be maintained in a test where thermal stress occurs, e.g., in the heat cycle test.
With use of the method in which a solvent with a low boiling point or a reactant type diluent was added, these components volatilized and decreased enormously in weight while being hardened through heat-pressing. Due to these volatile components, the base material either swelled or lost peel strength to the copper foils of the wiring.
Furthermore, when a dispersant was not added, it was a problem that particle forms were limited to reduce viscosity, and that a printed filling of via holes was difficult even with use of those which are rather low in viscosity since the viscosity raised under high shear during the printing process.